DESCRIPTION Animals have developed strategies to prevent the fusion of more than one sperm with the egg because polyspermic fertilization lead to abnormal development of the embryo. Our understanding of the molecular mechanisms that contribute to the block to polyspermy in mammals in incomplete, but has been studied in great detail in the frog Xenopus laevis. Two mechanisms that block polyspermy have been identified in X. laevis, and one of these involves a lectin-ligand interaction at outer surface of the egg envelope which prevents sperm from binding to and penetrating the envelope. The cortical granule lectin (CG lectin) involved has been cloned and sequenced. There is evidence in the EST database that a gene homologous to the CG lectin exists in humans and mice. My hypothesis is that a CG lectin homolog exists in the mammalian egg and participates in a lectin-ligand block to polyspermy in human and mouse eggs. To test this hypothesis, I will identify, clone and sequence, CG homologs from human and mouse cDNA libraries. CG homologs will be immunolocalized in unfertilized and activated cells from humans and mice. The blocking effects of pretreating salt-stored human and mice eggs with CG homologs will be examined by a zona pellucida penetration assay to determine if the homologs function in the block to polyspermy. The experiments I proposed will specifically test the hypothesis. If an evolutionary conserved lectin-.ligand block to polyspermy occurs in human eggs, a lectin-based contractive strategy could be developed.